Two types of the ultrasonic motor are conventionally known. One is a standing-wave type motor which employs a Langevin oscillator as a driving source. This type of motor is disclosed in U.S. Pat. No. 4,019,073. The other is a travelling-wave type motor in which a travelling wave is generated on a stator for driving a rotor provided on the stator. The travelling-wave motor generates smaller wear at the friction-transmission surface between the stator and the rotor, and is easier drive in a reverse-rotation direction in comparison with the standing-wave type motor. Such a travelling-wave motor is disclosed in U.S. Pat. No. 4,513,219 to Katsuma et al., U.S. Pat. No. 4,562,374 to Sashida and European Pat. Appl. Pub. No. 169297 of Tokushima. Katsuma et al. and Sashida disclose a travelling-wave motor employing a ling type of a piezoelectric member. This type of travelling-wave motor essentially consists of an annular vibrating body and a movable body provided thereon. The vibrating body has an annular piezoelectric vibrator thereon. The vibrating body is fixed to a base through a supporting mechanism. On the annular type piezoelectric vibrator, a gap portion with the length of half of the arc of an electrode is provided between two electrode groups. The travelling wave is vibrated by applying an AC signal having a phase difference of 90.degree. to the two groups.
Another type of travelling-wave motor employing a disk-shaped piezoelectric member is disclosed in the European Patent Application of Tokushima. In such motor, a stator is a disk-shaped elastic vibrating body having a toothlike circular protrusion. The vibrating body has a disk-shaped piezoelectric vibrator thereon. A movable body is provided on the protrusion of the vibrating body and has a central shaft for rotational guide. A pressure-regulating mechanism is provided on the central shaft for making suitable contact pressure between the vibrating body and the movable body to efficiently conduct the travelling wave component to the movable body. The vibrating body is supported and fixed on two circular protrusions formed on a base. The disk-shaped piezoelectric vibrator consists of a plurality of sector electrode interposed in such a manner that the piezoelectric vibrator slips by a half pitch of the arc length, the piezoelectric vibrator having an even number of electrodes in the circumferential direction in such a manner that each electrode has the same arc length.
In a travelling-wave motor of the type described above if the structure thereof is, so constituted that an annular vibrating body is included, the travelling wave is significantly damped to a certain degree due to a supporting mechanism since the flexible travelling wave which has been excited by a piezoelectric vibrator includes no nodal point of oscillation. As a result of this, electrical-mechanical transducing efficiency is deteriorated. If the structure of the travelling-wave motor is so constituted that a disc-type of vibrating body is included, an advantage is obtained in which the vibrating body can be fixed and supported by two places in the radial direction of the base by the base because the vibrating body is excited in a secondary oscillation mode in the radial direction of the vibrating body. However, the above-described deterioration in efficiency occurs due to the dispersion in the position of the nodal points and the magnitude of the supporting force. As the diameter and the thickness are decreased, the affection inevitably increases. Furthermore, since the disk type motor employs the movable body which is and, integrally formed with the central shafts, the shafts must be supported by means of bearing member provided in the movable body. If the ring type motor employs such movable body, a bearing member is required to be provided in the base. Furthermore, since the structure of a piezoelectric vibrator of the type described above is constructed in such a manner that the neighboring electrodes are polarized in a different direction with respect to each other, it is necessary to provide a proper gap between the electrodes for the purpose of preventing leaks or cracks and so on due to concentration of stress generated during polarization in a high electric field. Therefore, gaps of a proper width between, neighboring electrodes are provided. In general, a suitable width of the gap is considered to be substantially twice the thickness of the piezoelectric oscillator. These gaps raise a problem in that the phases of the travelling waves or the amplitude of the travelling waves deteriorate. Furthermore, since these gaps are the unsaturated portions left from polarization of the neighboring electrodes in different directions, the wavelengths of each travelling wave are non-uniform. Furthermore, when a device with a small diameter or a device of a low electricity consumption is intended to be realized, such structure in which most of the circumferential portion are occupied by input electrodes interrupt them.